In certain applications, nickel-cadmium storage cells are being replaced by nickel-hydride storage cells. Sealed nickel-hydride storage cells are alkaline storage cells having an aqueous electrolyte. The reagent, which is constituted by hydrogen, is stored in the mass of the hydridable alloy which is capable of absorbing large quantities thereof. Depending on whether the storage cell is being charged or discharged, such an alloy must be capable of taking up hydrogen and of restoring it at sufficient speed under normal operating conditions. Its electrochemical capacity must also be higher than that of cadmium, it must withstand corrosion in potassium hydroxide, and it must not be toxic.
Currently, the main alloys used are hydridable alloys of the AB.sub.5 type, derived from LaNi.sub.5. The compound LaNi.sub.5 has a reversible solid-gas hydrogen absorption capacity equivalent to 370 mAh/g. However, its plateau pressure, which is about 2 bars, is too high for it to be used in the negative electrode of a storage cell, for which the plateau pressure must lie in the range 0.01 bars to 1 bar. Furthermore, that alloy has insufficient resistance to corrosion in potassium hydroxide.
To reduce the cost of LaNi.sub.5 type alloys, lanthanum is generally replaced by misch metal, designated by Mm, whose composition is typically: EQU La.sub.0.25 to 0.35 Ce.sub.0.45 to 0.55 Nd.sub.0.10 to 0.20 Pr.sub.0.03 to 0.07.
By partially substituting nickel with aluminum and manganese, it is possible to lower the plateau pressure, and by partially substituting nickel with cobalt, it is possible to improve the resistance to corrosion of the alloy.
An alloy of the MmB.sub.5 type, based on misch metal Mm and satisfying the formula MmNi.sub.3.5 Mn.sub.0.5 Al.sub.0.3 Co.sub.0.7, is described in Japanese Patent Application JP-61 233 969. A storage cell implementing an electrode whose alloy has the above composition does not have the capacity that might be expected.